1. Field of the Invention
This invention, relates to plasma processing and in particular to plasma processing of devices.
2. Description of the Prior Art
Plasma discharges are extensively utilized in the fabrication of devices such as semiconductor devices and, in particular, silicon semiconductor devices. For example, plasma discharges in appropriate precursor gases are utilized to induce formation of a solid on a deposition substrate (One important embodiment of such a procedure is called plasma assisted chemical vapor deposition.) In a second plasma dependent procedure, species generated in a plasma are utilized to etch a substrate, e.g. a device substrate being processed which generally includes dielectric material, semiconductor material and/or material with metallic conductivity.
In plasma-assisted deposition procedures the desired solid is commonly formed by the reaction of a gas composition in a discharge. In one variation, reactive radical(s) formed in the plasma region either alone, or as mixed outside of the discharge region with a second gas, are flowed over a deposition substrate remote from the discharge to form the desired solid film. In another variation, the substrate is surrounded by a plasma which supplies charged species for energetic ion bombardment. The plasma tends to aid in rearranging and stabilizing the film provided the bombardment is not sufficiently energetic to damage the underlying substrate or the growing film.
In etching procedures, a pattern is typically etched into the substrate by utilizing a mask having openings corresponding to this pattern. This mask is usually formed by depositing a polymeric photosensitive layer, exposing the layer with suitable radiation to change the solubility of the exposed regions, and then utilizing the induced change in solubility to form the desired pattern through a solvation process.
For most present day device applications, it is desirable to produce anisotropic etching at an acceptable etch rate. (Acceptable etch rates depend upon the material to be removed and are generally those that remove at least 2% of the layer thickness in a minute. Anisotropic etching for the purpose of this description is an etch which undercuts the etch mask a distance less than one quarter the layer thickness.) The production of relatively vertical sidewalls during anisotropic etching allows higher packing densities for device structures. Additionally, the production of a relatively high etching rate leads to shorter processing times.
In one method of anisotropic etching, appropriate charged species generated in the plasma produce energetic ion bombardment that induces anisotropic etching. Various sources for producing the desired plasma discharge have been employed. For example, parallel plate reactors as described in C. J. Mogab, VLSI Technology, ed Sze at McGraw-Hill, N.Y. 1983, pgs. 303-345, and reactors having hexagonal electrodes as described in U.S. Pat. No. 4,298,443 dated Nov. 3, 1981 have been employed to induce anisotropic etching. Radio frequency resonators such as helical resonators have been used at pressures above 0.1 Torr as a source of etching species solely for isotropic etching. The species generated in the resonator are chemically reactive but have not demonstrated the momentum required for anisotropic etching.
As an alternative, a technique based on electron-cyclotron resonance (commonly referred to as ECR) discharges that generate high energy species for anisotropic etching has been described for the generation of ions at low pressure. (See Suzuki, et al. Journal of the Electrochemical Society 126, 1024 (1979).) However, the relatively high cost of an ECR is not entirely desirable. Additionally the etching of device structures suitable for 0.25 .mu.m devices has not been reported.